[new plugin] reimplement inlineDefs

[TrySound]

The code is taken from https://github.com/svg/svgo/pull/976, refactored with new api, covered types and simplified.

Plugin has no dependencies so can be used without changing.

const inlineDefs = require('./inlineDefs.js');

module.exports = {
  plugins: [
    'preset-default',
    inlineDefs
  ]
};

[itayganor]

Will this ever happen?

[strarsis]

@TrySound, @itayganor: Yes, dereference <use> plugin: https://github.com/svg/svgo/pull/1279